Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis­(phosphine) PBP pincer ligand. The previously reported complexes (PBP)­Ir­(Ph)­(Cl) (<b>1</b>) and (PBP)­Ir­(H)­(Cl) (<b>2</b>) were converted to the new complexes (PBP)­IrH₄ (<b>3</b>) and (PBP)­Ir­(Ph)­(H) (<b>4</b>). Complexes <b>3</b> and <b>4</b> serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs <i>tert</i>-butylethylene. The dicarbonyl complex (PBP)­Ir­(CO)₂ (<b>6</b>) was also synthesized, by the reaction of CO with either <b>3</b> or <b>4</b>. Intermediates (PB^PhP)­Ir­(H)­(CO)₂ (<b>5</b>) and (PBP)­IrH₂(CO) (<b>7</b>) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of <b>3</b> and <b>6</b>. Solid-state structures of <b>3</b> and <b>6</b> were determined by X-ray crystallography

Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis­(phosphine) PBP pincer ligand. The previously reported complexes (PBP)­Ir­(Ph)­(Cl) (<b>1</b>) and (PBP)­Ir­(H)­(Cl) (<b>2</b>) were converted to the new complexes (PBP)­IrH₄ (<b>3</b>) and (PBP)­Ir­(Ph)­(H) (<b>4</b>). Complexes <b>3</b> and <b>4</b> serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs <i>tert</i>-butylethylene. The dicarbonyl complex (PBP)­Ir­(CO)₂ (<b>6</b>) was also synthesized, by the reaction of CO with either <b>3</b> or <b>4</b>. Intermediates (PB^PhP)­Ir­(H)­(CO)₂ (<b>5</b>) and (PBP)­IrH₂(CO) (<b>7</b>) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of <b>3</b> and <b>6</b>. Solid-state structures of <b>3</b> and <b>6</b> were determined by X-ray crystallography

One-Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer Ligands and Their Nickel Complexes

A one-pot synthesis of arene-based PCP/PNP ligands has been developed. The reaction of 1,3-bis­(bromomethyl)­benzene or 2,6-bis­(bromomethyl)­pyridine with various chlorophosphines in acetonitrile afforded bis-phosphonium salts. These salts can then be reduced by magnesium powder to yield PCP or PNP ligands. In comparison to traditional synthetic methods for making PCP/PNP ligands involving the use of secondary phosphines, this new alternative method allows for the use of chlorophosphines, which are cheaper, safer to handle, and have a broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups. Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand to generate nickel complexes in high yields

Selective <i>ortho</i> C–H Activation of Pyridines Directed by Lewis Acidic Boron of PBP Pincer Iridium Complexes

Transition-metal mediated C–H functionalization has emerged as a powerful method in the chemistry relevant to the synthesis of pharmaceuticals, agrochemicals, and advanced materials. Because organic molecules typically contain multiple types of C–H bonds, selective C–H functionalization is a major ongoing challenge. C–H activation of heteroatom-containing organics has often been approached via the use of the directing effect, whereby the coordination to the basic heteroatom directs the reactive metal center to a specific C–H bond. We now report a different approach where the nitrogen donor in pyridine derivatives coordinates to an ancillary Lewis acidic boryl ligand directly attached to the metal (iridium) center, as opposed to the metal itself. This topology directs the iridium center to activate a different C–H bond than in the cases of directing donor coordination to the metal. Using this strategy, we demonstrate <i>ortho</i>-regiospecific C–H activation of pyridines and an example of the subsequent functionalization via C–C bond formation

Irreversible Hydrolysis of PCP-Supported Rhenium(V) Acetates

Complexes (PCP^R)­Re­(O)­(OAc)₂ [R = ⁱPr (<b>4a</b>) and ^tBu (<b>4b</b>); PCP = κ³-<i>P</i>,<i>C</i>,<i>P</i>-2,6-(R₂PCH₂)₂C₆H₃] undergo unexpected irreversible hydrolysis to yield (PCP^R)­Re­(O)­(OAc)­(OH) (<b>3a</b>/<b>3b</b>) and free AcOH. <b>3a</b> and <b>3b</b> are highly fluxional in solution, possibly via AcOH loss and the intermediacy of (PCP^R)­Re­(O)₂, which was isolated for R = ^tBu (<b>5b</b>)

Tandem Isomerization and C–H Activation: Regioselective Hydroheteroarylation of Allylarenes

The first Ni-promoted prototype reaction based on the tandem C–H activation of heteroarenes with alkene isomerization is demonstrated, leading to the branched hydroheteroarylation products. Simultaneously, the reaction selectivity can be chemically switched to linear adducts through Ni–Al tandem catalysis

Boryl/Borane Interconversion and Diversity of Binding Modes of Oxygenous Ligands in PBP Pincer Complexes of Rhodium

A series of Rh complexes derived from a PBP-type pincer ligand have been synthesized and characterized. It was previously reported that reaction of [(COD)­RhCl]₂ with 2,2′-bis­(diisopropylphino)­triphenylborane (<b>1</b>) resulted in a mixture of complexes containing a <i>Z</i>-type borane interaction (<b>2-Cl</b>), a boryl pincer (<b>3a-Cl</b>), and a η² binding of the B–Ph bond to Rh (<b>4-Cl</b>). In this work, we demonstrate that analogous complexes are accessible by replacement of chloride with potentially bidentate acetylacetonate, carboxylate, and trifluoromethanesulfonate ligands. In addition, a new type of isomer was observed in complexes with acetate and pivalate, where the carboxylate bridges between Rh and B (<b>3b-OAc</b>, <b>3b-OPiv</b>). All of these types of complexes are isomeric, and the preference for particular isomers for different anionic ligands varies. These isomers differ and are related by a change in the coordination mode of the oxygenous ligands and the migration of the Ph group between B and Rh

Facile Insertion of Rh and Ir into a Boron–Phenyl Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes

The unexpectedly facile insertion of Rh or Ir into a B–Ph bond (reversible for Rh) converts a borane/bis­(phosphine) precursor into a boryl/bis­(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen

Facile Insertion of Rh and Ir into a Boron–Phenyl Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes

The unexpectedly facile insertion of Rh or Ir into a B–Ph bond (reversible for Rh) converts a borane/bis­(phosphine) precursor into a boryl/bis­(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen

The Regioselective Switch for Amino-NHC Mediated C–H Activation of Benzimidazole via Ni–Al Synergistic Catalysis

We have disclosed a new mode of a chemically regioselective switch for C–H bond functionalization of benzimidazole derivatives via a cooperative effect invoked by Ni–Al bimetallic catalysis to create a steric requirement for obtaining the linear product of styrene insertion. Yet, excluding the AlMe₃ cocatalyst switches the reaction toward branch selectivity